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Old 6-Sep-2013, 2:14 PM   #67
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Multipath: STB-Converter Performance

The reception of WHAS-11 using the stack of long-Yagis was evaluated through the end of 2010 and into 2011.

Most of the time the signal strength seemed sufficient. It appeared that multipath was the main cause of occasional loss-of-lock in the daytime hours. Since multipath seemed likely as the cause, there was a possibility that differences in a DTV receiver’s equalizer could affect the situation.

Previous testing and TV viewing utilized a Channel Master CM-7000 set-top box. A comparison between two available converter boxes was made. The two other converters were Apex DT-250A and Magnavox TB110MWG. Each converter was connected to the antenna/TV combination and WHAS-11 was viewed for several minutes, then the CM-7000 was installed. This process was repeated multiple times over a period of two days. The visual display of loss-of-lock and picture pixelation was different, but there was no clear difference in overall performance.

Later in 2011, two other converters were tested. These were Digital Stream DTX-9900 and Zentih/Insignia NS/DXA-1. This time the tests were conducted in parallel, using a Holland GHS-2 splitter, both converters feeding separate monitors were operated from the same antenna. There was very little difference in visual performance. When a dropout occurred with one converter, nearly simultaneous a dropout appeared on the other converter. If there was any advantage, it went to the CM-7000. With the CM-7000, there seemed to be a very slight audio advantage, providing maybe a syllable more, either before dropout, or recovering early sometimes.

A search of literature, as to whether different converter boxes had appreciably different approaches to equalization, yielded the following report:

NAB/MSTV Digital Converter Box Evaluation – December 2008

In the above report, the performance of some set top boxes were evaluated. The Sansonic FT-300A appeared better able to handle both the longest delayed echos and the most advanced echos (+50 uS & - 50 uS).

Also, the “Pilot-nulled multipath threshold of visible (TOV)” test showed the Sansonic FT-300A to be 0.5 dB better than any of the other 6 converters tested.

The following image was drawn from data available in the above referenced report.

That 0.5 dB differential (left portion of above image, W Acq) for pilot nulling might not seem like much improvement. But if the pilot signal is lost, the system will lose lock. The pilot is located at the low frequency end of the 6 MHz DTV spectrum and is a fundamental synchronizing signal. A multipath signal could cause a frequency dependent null to severely attenuate the pilot. Recall, that 0.5 dB rejection of multipath can improve null depth up to about 6 dB. This means that the Sansonic unit was probably handling a worse pilot degradation than indicated by the 0.5 dB difference in applied signal.

A Sansonic FT-300A unit was purchased.

A simultaneous parallel comparison to the CM-7000 was arranged as described above. The CM-7000 lost lock a few times over the test period covering parts of several days, but the Sansonic unit provided solid reception.

The report above provided the identification of tuners and demodulators in the converters used in the test. Sansonic used a Microtune MT2131F tuner and an Auvitek AU8515AA demodulator. These components are different than any of the other test converters, they are also different than those used in the CM-7000.

Other testing confirmed that when a low noise moderate-to-high gain preamplifier is used, the tuner sensitivity is not a crucial factor in reception. So, the Sansonic tuner was probably not the critical part that contributed to better handling of multipath. The equalizer function is the likely reason for good performance and it is undoubtedly associated with the AU8515AA demodulator.

A search for more a commonly available converter that included the AU8515AA demodulator turned up Zinwell. Several Zinwell units were purchased including: ZAT-950A, ZAT-970A, newer versions labeled ZAT-970A on outside with ZAT-950 components inside. A visual inspection revealed all Zinwell units (made at that time) had the AU8515AA demodulator. Of the Zinwell units, some had can tuners (Sanyo) and some with silicon tuners (Microtune).

Simultaneous parallel tests of the Zinwell’s against the CM-7000 and the Sansonic were arranged as described above. The CM-7000 lost lock several times over the test period (covering parts of several days). The Sansonic unit and all the Zinwell units provided solid reception. Then, the Sansonic was compared against the Zinwell units, performance was identical.

A Zinwell unit was placed in service with the stack of 2 long Yagis.

Reception of WHAS-11 and WBNA-8 has been very satisfactory.

Attached Images
File Type: gif STB Compare WHAS-11.gif (3.7 KB, 5263 views)
File Type: gif Converter Pilot Null Test.gif (8.2 KB, 5318 views)
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